Iridium is a collection of 66 polar-orbiting satellites (it should have been 77, which is the atomic number of iridium, but some failed to reach orbit or work properly when sent aloft). They formed the basis of a satellite based mobile phone service, a technical marvel but a marketing and financial disaster. The company became insolvent in 2001 but the backers, including Motorola, have sold it as a going concern, and according to Iridium's website all is now well.
Iridium satellites are unusual in that for most of the time they are too faint to see, and at around 700km are in a higher orbit than most satellites seen from the ground. However, each satellite's construction includes an array of antennas that must be very carefully aligned for not only communicating with users on the ground but also for passing phone calls and data to and from other Iridium satellites. These antennas act as mirrors and if you are fortunate enough to be in the right place at the right time you will see a flash of sunlight directed at you as the beam sweeps past at 16,000 mph.
The brightness depends on how close you are to the centre of the beam. A brightness of magnitude -2 (similar to Jupiter) seems common for being about 20km from the centre, -5 (slightly brighter than Venus) if 10km, and -8 for ground zero. This is an extraordinary brightness, 25 times brighter than Venus ever gets, and potentially puts Iridium as the brightest thing in the night sky apart from the moon. In addition, you are also seeing something at probably the greatest distance you are ever likely to see a man-made object.
To see the flare from any one satellite is a rare event, but because there are 66 of them the odds are quite favourable. It should be possible to see one, and sometimes two events in an evening.
Two websites tell you more about them...
Professional astronomers hate Iridium events because they can ruin exposures and wreck sensitive instruments. They use predictions to specifically avoid catching them on film or CCD, but for the likes of us they are something well worth trying to photograph or catch on video camera.
The Heavens Above predictions are detailed enough to allow a camera to be correctly positioned and the shutter opened on B in anticipation of the event. For an interesting shot use a regular lens with the aperture as wide as possible, open the shutter 30 seconds before the expected time of the flash, and keep it open at least as long afterwards. This will show the trail of the satellite as it brightens then fades and be long enough to show a good number of stars, which themselves will appear as short trails owing to the Earth's rotation.
Anyone in line for a magnitude -7 or -8 flare should make an extra effort and be rewarded with seeing and capturing the brightest thing in the night sky apart from the moon.
|Sightings in 2001|
|Sightings in 2002|
|Sightings in 2003|